The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Japan's whaling organisation has accused Australia of assisting the "sabotage" of its hunt by Sea Shepherd by giving the environmental group a safe haven.

But Dr Brown says the government needed to ensure the freedom of Mr Watson, who remains at sea to avoid arrest through Interpol on Japanese charges of obstructing that country's business activities.

"The Australian government should be guaranteeing that his freedoms will be upheld if he comes ashore here and that this belligerent and bullying and illegal pursuit of whale defenders by the Japanese government will not be effective on Australian soil," Dr Brown said.

A director of Sea Shepherd Australia since quitting the Senate last year, Dr Brown described Mr Watson as "one of the great environmentalists of recent times".

"We have not heard from the Australian Attorney-General (Mark Dreyfus) that he won't be arrested," he said.

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The attorney-general's department does not comment publicly on extradition requests.

Dr Brown called on the federal government and opposition to commit to sending a navy or customs ship to the Southern Ocean when Japan's whaling season begins again next summer.

He said the whaling fleet was flouting a Federal Court injunction banning the killing of whales in Australian waters.

"It's an enormously important issue to the Australian public and we need to know in the run to the election which of the big parties ... is going to use Australia's capability of protecting its backyard and the whales that are in it."

Prime Minister Julia Gillard has ruled out such a move.

But late on Tuesday opposition environment spokesman Greg Hunt told Fairfax Media the coalition would send a customs ship to patrol the area.

The Japanese fleet returned home this week having caught 103 minke whales, well short of their target of more than 1000 whales of three species under an international whaling convention loophole that allows some whales to be killed for scientific research.